The scale on a map is 10cm : 8km. If the distance between two cities is 80km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 10cm on the map corresponds to an actual distance of 8km. An actual distance of 80km is the same as 10 $\cdot$ 8km. The distance between the two cities on the map is 10 $\cdot$ 10cm, or 100cm.